Conventional methods of forming TCOs on glass substrates require high glass substrate temperatures. Such methods include chemical pyrolysis where precursors are sprayed onto the glass substrate at approximately 400 to 500 degrees C., and vacuum deposition where the glass substrate is kept at about 150 to 300 degrees C. It is often not desirable to require such high glass substrate temperatures for TCO deposition processing.
Sputter deposition of a TCO at approximately room temperature would be desirable, given that most float glass manufacturing platforms are not equipped with in-situ heating systems. Thus, it would be an achievement in the art if a technique for sputter-depositing TCOs could be realized that would result in a sufficiently conductive film.
A limitation of low-temperature sputter deposition of TCOs is the low atom mobility on the glass substrate. This limits the ability of species to find their optimal positions, thereby reducing film quality due to less than desirable crystallinity. The low atom mobility is particularly problematic for dopant atoms which are often introduced to a stoichiometric TCO to produce free electrons. At low deposition temperatures, the dopant atoms tend to cluster such that their efficiency becomes reduced.
In certain example embodiments of this invention, one or more of the above problems are addressed by sputter-depositing a TCO inclusive film at a low temperature (e.g., less than about 150 degrees C., more preferably less than about 100 degrees C., and possibly at approximately room temperature) by sputter-depositing both a primary dopant and a co-dopant. The use of both the primary dopant and the co-dopant in depositing (e.g., sputter-depositing) the TCO inclusive film prevents or reduces the formation of compensating native defects in a wide-bandgap semiconductor material during the impurity introduction by controlling the Fermi level at or proximate the edge of the growth.
Immediately after being captured by surface forces, atoms start to migrate and follow the charge neutrality principle. The Fermi level is lowered at the growth edge by the addition of a small amount of acceptor impurity (such as Ag) so it prevents or reduces the formation of the compensating (e.g., negative in this case) species, such as zinc vacancies. After the initial stage of the semiconductor layer formation, the mobility of atoms is reduced and the probability of the point defect formation is primarily determined by the respective energy gain. Silver atoms for example in this particular example case tend to occupy interstitial sites where they play a role of predominantly neutral centers, forcing Al atoms to the preferable zinc substitutional sites, where Al plays the desired role of shallow donors, thus eventually raising the Fermi level. In addition, the provision of the co-dopant promotes declustering of the primary dopant, thereby freeing up space in the metal sublattice and permitting more Al to function as a charge carrier so as to improve conductivity of the film. Accordingly, the use of the co-dopant permits the primary dopant to be more effective in enhancing conductivity of the TCO inclusive film, without significantly sacrificing visible transmission characteristics. Furthermore, the use of the co-dopant improves crystallinity of the TCO inclusive film and thus the conductivity thereof, and grain size may also increase which can lead to increased mobility.
In certain example embodiments of this invention, the TCO film may be sputter-deposited on a glass substrate (either directly or indirectly) at approximately room temperature. In alternative embodiments, it is possible to pre-heat the glass substrate prior to the sputter-deposition of the TCO film. In yet another embodiment, it is possible to heat the glass substrate with the TCO layer after the deposition thereof, e.g., during a glass tempering and/or heat strengthening step.
In an example embodiment, a zinc oxide based film includes Al as a primary dopant and Ag as a co-dopant. In this respect, the Al is the primary charge provider. It has surprisingly been found that the introduction of Ag to ZnAlOx promotes declustering of the Al and permits more Al to function as a donor thereby improving crystallinity and conductivity of the film. In the case of introducing Ag as the co-dopant (acceptor) into ZnO, Ag facilitates the introduction of the primary donor dopant (Al). Certain example embodiments of this invention may also use the ability of silver to promote the uniform or substantially uniform distribution of donor-like dopants in wide-bandgap II-VI compounds, thereby allowing one to increase the effective dopant concentration in a poly-crystalline film.
While silver is used as a co-dopant in certain example embodiments of this invention, it is possible to use another Group IB, IA or V element such as Cu or Au instead of or in addition to silver as the co-dopant.
In certain example embodiments of this invention, there is provided a method of making a coated article including a transparent conductive film, the method comprising: providing a glass substrate; sputtering at least one target comprising each of zinc, aluminum and silver in an atmosphere comprising oxygen so as to form a transparent conductive film on the glass substrate.
In other example embodiments of this invention, there is provided a method of making a coated article including a transparent conductive film, the method comprising: providing a substrate; sputtering at least one target comprising each of zinc, a primary metal dopant, and a co-dopant of at least one Group IB, IA or V element, wherein the sputtering is performed in an atmosphere comprising oxygen so as to form a transparent conductive film comprising zinc, oxygen, the primary metal dopant and the co-dopant on the substrate.
In still further example embodiments of this invention, there is provided a coated article comprising: a transparent conductive film provided on a glass substrate; and wherein the transparent conductive film comprises zinc aluminum oxide that is doped with silver in order to enhance electrical properties of the film.
In other example embodiments of this invention, there is provided a coated article comprising: a transparent conductive film provided on a substrate; and wherein the transparent conductive film comprises MAl oxide that is doped with silver in order to enhance electrical properties of the film. The metal M may be Zn or the like in certain example embodiments.